Embolus to the left main coronary artery is extremely rare, and no previous report has focused on fibrin-platelet emboli to this artery. One report described a myocardial embolus to this artery. During the past 2 years, we studied 2 men, aged 37 and 48 years, who had fibrin-platelet emboli to the left main coronary artery, causing sudden death in each. In both, the major coronary arteries were essentially free of atherosclerotic plaque. The source of the embolus in 1 was unknown; in the second, the source was peripheral venous thrombi which crossed the atrial septum (paradoxic embolus). A third patient, a 49-year-old woman studied in 1970, had an embolus to the left main coronary artery during left-sided cardiac catheterization. The latter patient had severe narrowing by atherosclerotic plaque of all the major epicardial coronary arteries. Thus, embolus to the left main coronary artery is rare but usually fatal.